Lessons on Soil | Page 7

soil. The muddy liquid is obtained as before, it takes a long time to settle, but in the end it gives a sediment so much like that from the soil, except in colour, that we shall be safe in saying that the sediments in the jars contain the clay from the soil. And thus we have been able to separate the sticky part of the soil--the clay--from the gritty or sandy part which is not at all sticky. We may even be able to find out something more. If we leave the soil sediment and the clay sediment on separate tin lids to dry, and then examine them carefully we may find that the {7} soil sediment is really a little more gritty than the clay. Although it contains the clay it also contains something else.
When the experiment is made very carefully in a proper way this material can be separated from the pure clay. It is called silt, but really there are a number of silts, some almost like clay and some almost like sand; they shade one into the other.
If there is enough grit it should be weighed: we obtained 14 decigrams of grit from 10 grams of our top soil and 17 decigrams from 10 grams of bottom soil. We cannot separate the clay from the silt, but when this is done in careful experiments it is found that the subsoil contains more clay than the top soil. We should of course expect this because we have found that the subsoil is more sticky than the top soil. These results are put into the columns as before so that we can now see at once how much of our soil is water, how much can burn away, how much is grit, and how much is clay and other things.
What would have happened if the sample had been dug out during wetter or drier weather? The quantity of water would have been different, but in other respects the soil would have remained the same. It is therefore best to avoid the changes in the amount of water by working always with 10 grams of dried soil. The results we obtained were:--
Top soil Subsoil Weight of dry soil before burning ... 100 100 decigrams " " " after " ... 92 97 " --- --- The part that burned away weighed ... 8 3 Weight of grit from 10 grams of dried soil 17 19 "
The columns are given in Fig. 3.
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[Illustration: Fig. 3. Columns showing what 100 parts of dried soil and subsoil were made of]
Summary. The experiments made so far have taught us these facts:--
1. Soil contains water, grit or sand, silt, clay, a part that burns away, and some white chalky specks.
2. The top layer of soil to a depth of about eight inches is different from the soil lying below, which is called the subsoil. It is less sticky, easier to dig, and darker in colour. It contains more of the material that burns away, but less clay than the subsoil.
3. When soil is dried it is not sticky but hard or crumbly; as soon as it is moistened it changes back to what it was before. But when soil is burnt it completely alters and can no longer be changed back again.

[1] See p. xiv for explanation of the figures in square brackets.

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CHAPTER II
MORE ABOUT THE CLAY
Apparatus required.
Clay, about 6 lbs.; a little dried, powdered clay; sand, about 6 lbs. Six glass jars or cylinders [2]. Six beakers [1]. Six egg-cups [1]. Six funnels and stands [2]. Six perforated glass or tin disks [2]. Six glass tubes [2]. Two tubulated bottles fitted with corks. Some seeds. Six small jars about 2 in. x 1 in. [2]. Bricks. The apparatus in Fig. 9. Pestle and mortar.
We have seen in the last chapter that clay will float in water and only slowly settles down. Is this because clay is lighter than water? Probably not, because a lump of clay seems very heavy. Further, if we put a small ball of clay into water it at once sinks to the bottom. Only when we rub the clay between our fingers or work it with a stick--in other words, when we break the ball into very tiny pieces--can we get it to float again. We therefore conclude that the clay floated in our jars (p. 6) for so long not because it was lighter than water, but because the pieces were so small.
Clay is exceedingly useful because of its stickiness. Dig up some clay, if there is any in your garden, or procure some from a brick works. You can mould it into any shape you like, and the purer the clay the {10} better it acts. Enormous quantities of clay are used for making bricks. Make some